Cytokinin oxidase promoter from maize

ABSTRACT

The present invention provides compositions and methods for regulating expression of nucleotide sequences in a plant. Compositions and methods include expression cassettes and transformed plants and provide for downregulation of cytokinin oxidase in a plant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/051,893, filed Mar. 20, 2008, to be issued as U.S. Pat. No.7,741,534, which is a divisional of U.S. patent application Ser. No.11/074,144, filed Mar. 7, 2005 and now U.S. Pat. No. 7,371,925, which isa continuation-in-part of U.S. patent application Ser. No. 10/109,488,filed Mar. 28, 2002 and now U.S. Pat. No. 6,921,815, which claims thebenefit of and incorporates by reference U.S. Provisional ApplicationNo. 60/279,805, filed Mar. 29, 2001.

FIELD OF THE INVENTION

The present invention relates to the field of plant molecular biology,more particularly to regulation of gene expression in plants.

BACKGROUND OF THE INVENTION

Expression of heterologous DNA sequences in a plant host is dependentupon the presence of an operably-linked promoter that is functionalwithin the plant host. Choice of the promoter sequence will determinewhen and where within the plant the heterologous DNA sequence isexpressed. Where continuous expression is desired throughout the cellsof a plant, constitutive promoters are utilized. In contrast, where geneexpression in response to a stimulus is desired, inducible promoters arethe regulatory elements of choice. Where expression in specific tissuesor organs is desired, tissue-preferred promoters are used. That is,these promoters can drive expression in specific tissues or organs.Additional regulatory sequences upstream and/or downstream from the corepromoter sequence can be included in expression cassettes oftransformation vectors to bring about varying levels of expression ofheterologous nucleotide sequences in a transgenic plant. See, forexample, U.S. Pat. No. 5,850,018.

Regulatory sequences may also be useful in controlling temporal and/orspatial expression of endogenous DNA. For example, specialized tissuesare involved in fertilization and seed development. Identification ofpromoters which are active in these seed tissues is of interest.

In grain crops of agronomic importance, seed formation is the ultimategoal of plant development. Seeds are harvested for use in food, feed,and industrial products. The quantities and proportions of protein, oil,and starch components in those seeds determine their utility and value.

The timing of seed development is critical. Environmental conditions atany point prior to fertilization through seed maturation may affect thequality and quantity of seed produced. In particular, the first 10 to 12days after pollination (the lag phase) are critical in maize seeddevelopment. Several developmental events during the lag phase areimportant determinants of the fate of subsequent seed growth anddevelopment. (Cheikh, N. et al., Plant Physiology 106:45-51 (1994))Therefore, a means to influence plant development, particularly inresponse to stress during this phase of growth, is of interest.Identification of a promoter sequence active in tissues of developingseeds exposed to abiotic stresses would be useful.

Specialized plant tissues are central to seed development. Followingfertilization, developing seeds become sinks for carbon translocated viathe phloem from sites of photosynthesis. However, developing cerealseeds have no direct vascular connections with the plant; instead, ashort-distance transport mechanism operates to move the assimilates fromthe vascular tissues to the endosperm and embryo. For example, in maize,photosynthate enters the seed via the pedicel; in wheat, via thenucellar projection and the aleurone layer. It is possible that thisshort-distance assimilate pathway between the phloem and the endospermcan operate to regulate the rate of sucrose transport into the grain.(Bewley, J. D., and M. Black. Seeds: Physiology of Development andGermination. N.Y., Plenum Press, 1985. pp. 38-39) Therefore, a promoteractive in gene expression within these specialized tissues, such as thepedicel, may have significant effects on grain development.

During rapid seed growth, sucrose is unloaded passively from the phloeminto the apoplast of the pedicel parenchyma and inverted to hexosesugars by a cell-wall-bound acid invertase. The hydrolysis of sucrose inthe apoplast maintains a favorable gradient for continued unloading fromthe phloem and provides hexoses that are taken up by the basal endospermcells. It has been shown that seeds induced to abort, in vitro, haveonly low levels of invertase activity in the pedicel. (Hanft, J. M. etal. (1986) Plant Physiol. 81:503-510)

Water stress to the plant around anthesis often results in seed abortionor restricted development. Studies suggest that sucrose continues tounload from the phloem at low ovary water potential, but it accumulatesin the symplasm and apoplasm of the pedicel because of low invertaseactivity. (Zinselmeier, C., et al., (1995) Plant Physiol. 107:385-391)This conclusion is supported by the findings of Miller and Chourey(Plant Cell 4: 297-305 (1992)), who showed that developmental failure ofminiature-1 seeds of maize was linked to lack of invertase activity inthe pedicel tissue during the early stages of seed development.

Other specialized plant tissues are also closely involved in thecritical processes of fertilization and seed development. For example,in maize, the carpels, which make up the ovary wall, become thepericarp, a tough, protective outer seed covering. The scutellum, alongwith the endosperm, is involved in translocation of assimilates to thedeveloping embryo. The aleurone, the surface layer of endosperm cells,develops to serve as a source of enzymes necessary in germination.(Kiesselbach, T. A. The Structure and Reproduction of Corn. N.Y., ColdSpring Harbor Press, 1999)

To achieve yield stability in high-density plantings, under droughtconditions, or in other adverse environments, modification ofcarbohydrate metabolism during early ear and kernel development may bedesirable. Effective control of genes involved in carbohydratemetabolism is dependent on identification and use of a promoter withhigh levels of tissue and temporal specificity. Specifically desiredexpression targets include pedicel, pericarp, and nucellus tissue duringa period 14 days before pollination to 14 days after pollination.

In light of the important contributions of these specialized seedtissues to proper grain development, identification of a promotersequence affecting gene expression in these tissues would be useful.Further, it would be desirable to identify a promoter sequence active inthese specific tissues at appropriate, critical times. Even moredesirable would be the identification of a promoter sequence active inthese specific tissues at appropriate, critical times, which is notnegatively affected by environmental stress to the plant.

The maize Glb1 gene encodes globulin-1, a major embryo storage protein.(Kriz, A. L., et al. (1986) Plant Physiol. 82:1069-1075) Glb1 isexpressed in the developing maize seed during embryo development.(Belanger, F. C., et al. (1989) Plant Physiol. 91:636-643) The promoterregion of Glb1 has been identified, cloned, and introduced into tobaccoplants by Agrobacterium-mediated transformation. (Liu, S., et al. (1996)Plant Cell Reports 16:158-162) The transformed plants demonstrate thatthe Glb1 promoter has desirable temporal and tissue specificity.However, the Glb1 promoter is positively regulated by abscisic acid(ABA). (Kriz, A. L., et al. (1990) Plant Physiol. 92:538-542; Paiva, R.,et al., (1994) Planta 192:332-339) Levels of the plant hormone ABA areknown to fluctuate under conditions of cold or desiccation. (Himmelbach,A., et al. (1998) Phil. Trans. R. Soc. Lond. 353:1439-1444) Thus, theactivity of the Glb1 promoter can be differentially affected byenvironmental stress.

A maize cytokinin oxidase gene has been isolated and sequenced (GenBankentry AF044603). Cytokinin oxidase inactivates cytokinins, members of aclass of plant hormones important in the control of cell division and inregulation of plant growth and structure. Elevated cytokinin levels areassociated with the development of seeds in higher plants; exogenouscytokinin application has been shown to directly correlate withincreased kernel yield in maize. Thus, control of the level of cytokininoxidase has been suggested as a tool in improving grain yield.Manipulation of cytokinin oxidase activity has also been proposed as ameans to achieve greater disease resistance or other improved plantcharacteristics. (See WO 99/06571, herein incorporated by reference.)

However, a novel and heretofore undescribed utility of the isolatedcytokinin oxidase gene is as a source of a promoter sequence withspatial and temporal specificity and which may be induced by cytokinins.A full-length promoter sequence of the isolated maize cytokinin oxidasegene, and functional fragments and variants thereof, and the use of suchsequences with heterologous nucleotide sequences of interest, aredescribed in the present invention. Unless otherwise specified, thenotation “ckx1-2” in reference to the subject promoter includes SEQ IDNO: 1, SEQ ID NO: 4, and any functional fragments or variants thereof.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel nucleotidesequence for modulating gene expression in a plant.

It is a further object of the present invention to provide an isolatedpromoter capable of driving transcription in a tissue-preferred manner.

It is a further object of the present invention to provide an isolatedpromoter sequence responsive to the presence of cytokinin.

It is a further object of the present invention to provide a method ofimproved control of an endogenous or exogenous product in a transformedplant.

It is a further object of the present invention to provide a method foreffecting useful changes in the phenotype of a transformed plant.

It is a further object of the present invention to provide a method forproducing a novel product in a transformed plant.

It is a further object of the present invention to provide a method forproducing a novel function in a transformed plant.

It is a further object of the present invention to provide a method formodulating the timing or rate of development of the seed of atransformed plant.

It is a further object of the present invention to provide a method forregulating the accumulation of photosynthetic products in the developingseed of a transformed plant.

It is a further object of the present invention to provide a method forregulating the production of phytohormones involved in seed development.

It is a further object of the present invention to provide a method forregulating the level of cytokinin oxidase activity within a plant.

It is a further object of the present invention to provide a method forregulating the cell cycle machinery of seeds during their development.

Therefore, in one aspect, the present invention relates to an isolatednucleic acid comprising a member selected from the group consisting of:

-   -   a) nucleic acids capable of driving expression in the carpel,        pericarp, pedicel, glume, nucellus, endosperm, vascular tissue,        or pedicel-forming region of a developing seed; in root tissue,        especially vascular tissue; in stalk tissue; and in premeiotic        anther tissue;    -   b) nucleic acids comprising a functional variant or fragment of        at least 20 contiguous nucleotides of the sequence set forth in        SEQ ID NO.: 4;    -   c) the nucleic acid sequence of SEQ ID NO.: 4; and    -   d) nucleic acids that hybridize to any one of a), b), or c)        under stringent conditions, wherein stringent conditions include        hybridization at 42° C. in a solution of 50% (w/v) formamide,        6×SSC, 0.5% SDS, 100 ug/ml salmon sperm, washed with 0.5% SDS        and 0.1×SSC at about 65° C. for 30 minutes and repeated.

In other aspects, the present invention relates to expression cassettescomprising the promoter operably linked to a nucleotide sequence,vectors containing said expression cassette, and plants stablytransformed with at least one said expression cassette.

In a further aspect, the present invention relates to a method formodulating expression in the seed, root, stalk, or vascular tissue of astably transformed plant comprising the steps of (a) transforming aplant cell with an expression cassette comprising the promoter of thepresent invention operably linked to at least one nucleotide sequence;(b) growing the plant cell under plant growing conditions and (c)regenerating a stably transformed plant from the plant cell wherein saidlinked nucleotide sequence is expressed in the seed, root, stalk, orvascular tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Characterization of transgenic maize plants expressing a fusionof the ckx1-2 promoter (SEQ ID NO: 1) and the β-glucuronidase (GUS)gene.

(A) GUS staining of a leaf section showing strong labeling in a largevascular strand (LVS) and small vascular strands (SVS). Bar=250 μm.

(B) Close-up of a leaf section showing GUS staining in the protoxylem(px) and bundle sheath cells (bs) of a large vascular strand. Note theabsence of staining in the phloem zone (ph). Bar=170 μm.

(C) GUS staining of a coleoptile cross-section showing strong labelingin the vascular bundles (VB). Bar=3 mm.

(D) Close-up of a transverse coleoptile section showing intense labelingin the vascular bundles. Bar=500 μm.

(E) Staining of lateral root showing GUS staining in the vascularbundle. Bar=250 μm.

(F) Staining of a primary root showing activity in emerging lateralroot. Bar=250 μm.

s=stomata, c=cortex, m×=metaxylem, p=parenchyma

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, a nucleotide sequence is provided thatfavors initiation of transcription in specific tissues, includingtissues of developing seed; roots, especially root vascular tissue;stalk tissue; and premeiotic anther tissue. The sequence of theinvention comprises transcriptional initiation regions associated withsaid tissues. Thus, the compositions of the present invention comprise anovel nucleotide sequence for a plant promoter, more particularly atissue-preferred promoter.

By “seed” or “kernel” is intended to include the grain or ripened ovuleof a plant, or more broadly, a propagative plant structure. The terms“seed” and “kernel” are used interchangeably herein.

“Operably linked,” as used herein, includes reference to a functionallinkage between a promoter and a second sequence, wherein the promotersequence initiates and mediates transcription of the DNA correspondingto the second sequence. Generally, operably linked means that thenucleic acid sequences being linked are contiguous and, where necessaryto join two protein coding regions, contiguous and in the same readingframe. An endogenous promoter is operably linked to the endogenouscoding region which it regulates.

As used herein, the term “plant” includes reference to whole plants andtheir progeny; plant cells; plant parts or organs, such as embryos,pollen, ovules, seeds, flowers, kernels, ears, cobs, leaves, husks,stalks, stems, roots, root tips, anthers, silk and the like. Plant cell,as used herein, further includes, without limitation, cells obtainedfrom or found in: seeds, suspension cultures, embryos, meristematicregions, callus tissue, leaves, roots, shoots, gametophytes,sporophytes, pollen, and microspores. Plant cells can also be understoodto include modified cells, such as protoplasts, obtained from theaforementioned tissues. The class of plants which can be used in themethods of the invention is generally as broad as the class of higherplants amenable to transformation techniques, including bothmonocotyledonous and dicotyledonous plants. Particularly preferred plantare maize, canola, and soybean.

By “heterologous nucleotide sequence” is intended a sequence that is notnaturally occurring with the promoter sequence. While this nucleotidesequence is heterologous to the promoter sequence, it may be homologous(native) or heterologous (foreign) to the plant host.

By “promoter” is intended a regulatory region of DNA usually comprisinga TATA box capable of directing RNA polymerase II to initiate RNAsynthesis at the appropriate transcription initiation site for aparticular coding sequence. A promoter can additionally comprise otherrecognition sequences generally positioned upstream or 5′ to the TATAbox, referred to as upstream promoter elements, which influence thetranscription initiation rate. It is recognized that having identifiedthe nucleotide sequences for the promoter region disclosed herein, it iswithin the state of the art to isolate and identify further regulatoryelements in the 5′ untranslated region upstream from the particularpromoter region identified herein. Thus the promoter region disclosedherein is generally further defined by comprising upstream regulatoryelements such as those responsible for tissue and temporal expression ofthe coding sequence, enhancers and the like. In the same manner, thepromoter elements which enable expression in the desired tissue such asthe seed can be identified, isolated, and used with other core promotersto confirm seed-preferred expression. Promoter elements, such as thoseconferring cytokinin inducibility, may also be identified and isolatedfor use with other core promoters.

The isolated promoter sequence of the present invention can be modifiedto provide for a range of expression levels of the heterologousnucleotide sequence. Less than the entire promoter region can beutilized and the ability to drive tissue-preferred expression retained.However, it is recognized that expression levels of mRNA can bedecreased with deletions of portions of the promoter sequence. Thus, thepromoter can be modified to be a weak or strong promoter. Generally, by“weak promoter” is intended a promoter that drives expression of acoding sequence at a low level. By “low level” is intended levels ofabout 1/10,000 transcripts to about 1/100,000 transcripts to about1/500,000 transcripts. Conversely, a strong promoter drives expressionof a coding sequence at a high level, or at about 1/10 transcripts toabout 1/100 transcripts to about 1/1000 transcripts. Generally, at leastabout 20 nucleotides of an isolated promoter sequence will be used todrive expression of a nucleotide sequence.

It is recognized that to increase transcription levels, enhancers can beutilized in combination with the promoter regions of the invention.Enhancers are nucleotide sequences that act to increase the expressionof a promoter region. Enhancers are known in the art and include theSV40 enhancer region, the 35S enhancer element, and the like.

By “tissue-preferred” promoter is meant a sequence which preferentiallyinitiates transcription in certain tissues, such as leaves, roots, orseeds. A tissue-preferred promoter also may drive expression in certaintissues types in one or more organs; for example, in vascular tissues ofroots or leaves.

By “seed-preferred” is intended favored expression in the seed,including at least one of embryo, seed or kernel, pericarp, endosperm,nucellus, aleurone, pedicel, and the like.

The term “isolated” refers to material, such as a nucleic acid or aprotein, which is: (1) substantially or essentially free from componentswhich normally accompany or interact with it as found in its naturalenvironment. The isolated material optionally comprises material notfound with the material in its natural environment; or (2) if thematerial is in its natural environment, the material has beensynthetically altered or synthetically produced by deliberate humanintervention and/or placed at a different location within the cell. Thesynthetic alteration or creation of the material can be performed on thematerial within or apart from its natural state. For example, anaturally-occurring nucleic acid becomes an isolated nucleic acid if itis altered or produced by non-natural, synthetic methods, or if it istranscribed from DNA which has been altered or produced by non-natural,synthetic methods. The isolated nucleic acid may also be produced by thesynthetic re-arrangement (“shuffling”) of a part or parts of one or moreallelic forms of the polynucleotide of interest. Likewise, anaturally-occurring nucleic acid (e.g., a promoter) becomes isolated ifit is introduced to a different locus of the genome. Nucleic acids whichare “isolated,” as defined herein, are also referred to as“heterologous” nucleic acids.

Methods for isolation of promoter regions are well known in the art.See, for example, U.S. Pat. Nos. 6,797,859; 6,720,480; and 6,617,498,hereby incorporated by reference.

One sequence for the promoter region of the present invention is setforth in SEQ ID NO.: 4.

The promoter region of the invention may be isolated from any plant,including, but not limited to maize (Zea mays), canola (Brassica napus,Brassica rapa ssp.), alfalfa (Medicago sativa), rice (Oryza sativa), rye(Secale cereale), sorghum (Sorghum bicolor, Sorghum vulgare), sunflower(Helianthus annuus), wheat (Triticum aestivum), soybean (Glycine max),tobacco (Nicotiana tabacum), potato (Solanum tuberosum), peanut (Arachishypogaea), cotton (Gossypium hirsutum), sweet potato (Ipomoea batatus),cassaya (Manihot esculenta), coffee (Cofea spp.), coconut (Cocosnucifera), pineapple (Ananas comosus), citrus trees (Citrus spp.), cocoa(Theobroma cacao), tea (Camellia sinensis), banana (Musa spp.), avocado(Persea americana), fig (Ficus casica), guava (Psidium guajava), mango(Mangifera indica), olive (Olea europaea), oat (Avena sativa),vegetables, ornamentals, and conifers. Preferably, plants include maize,rice, soybean, sunflower, safflower, canola, wheat, rye, alfalfa, andsorghum.

Promoter sequences from other plants may be isolated according towell-known techniques based on their sequence homology to the promotersequence set forth herein. In these techniques, all or part of the knownpromoter sequence is used as a probe which selectively hybridizes toother sequences present in a population of cloned genomic DNA fragments(i.e. genomic libraries) from a chosen organism. Methods are readilyavailable in the art for the hybridization of nucleic acid sequences.

The entire promoter sequence or portions thereof can be used as a probecapable of specifically hybridizing to corresponding promoter sequences.To achieve specific hybridization under a variety of conditions, suchprobes include sequences that are unique and are preferably at leastabout 10 nucleotides in length, and most preferably at least about 20nucleotides in length. Such probes can be used to amplify correspondingpromoter sequences from a chosen organism by the well-known process ofpolymerase chain reaction (PCR). This technique can be used to isolateadditional promoter sequences from a desired organism or as a diagnosticassay to determine the presence of the promoter sequence in an organism.Examples include hybridization screening of plated DNA libraries (eitherplaques or colonies; see e.g. Innis et al., eds., (1990) PCR Protocols,A Guide to Methods and Applications, Academic Press).

Use of in situ hybridization preserves cellular morphology forsubsequent interpretation and analysis. The following articles providean overview of the various hybridization assay formats: Singer et al.,Biotechniques 4(3): 230-250 (1986); Haase et al., Methods in Virology,Vol. VII, pp. 189-226 (1984); Wilkinson, The theory and practice of insitu hybridization in: In situ Hybridization, D. G. Wilkinson, Ed., IRLPress, Oxford University Press, Oxford; and Nucleic Acid Hybridization:A Practical Approach, Hames, B. D. and Higgins, S. J., Eds., IRL Press(1987).

The terms “stringent conditions” or “stringent hybridization conditions”include reference to conditions under which a probe will hybridize toits target sequence, to a detectably greater degree than to othersequences (e.g., at least 2-fold over background). Stringent conditionsare target-sequence-dependent and will differ depending on the structureof the polynucleotide. By controlling the stringency of thehybridization and/or washing conditions, target sequences can beidentified which are 100% complementary to a probe (homologous probing).Alternatively, stringency conditions can be adjusted to allow somemismatching in sequences so that lower degrees of similarity aredetected (heterologous probing). Generally, probes of this type are in arange of about 1000 nucleotides in length to about 250 nucleotides inlength.

An extensive guide to the hybridization of nucleic acids is found inTijssen, Laboratory Techniques in Biochemistry and MolecularBiology—Hybridization with Nucleic Acid Probes, Part I, Chapter 2“Overview of principles of hybridization and the strategy of nucleicacid probe assays”, Elsevier, New York (1993); and Current Protocols inMolecular Biology, Chapter 2, Ausubel, et al., Eds., Greene Publishingand Wiley-Interscience, New York (1995). See also Sambrook et al. (1989)Molecular Cloning: A Laboratory Manual (2nd ed. Cold Spring HarborLaboratory, Cold Spring Harbor, N.Y.).

In general, sequences that correspond to the promoter sequence of thepresent invention and hybridize to the promoter sequence disclosedherein will be at least 50% homologous, 70% homologous, and even 85%homologous or more with the disclosed sequence. That is, the sequencesimilarity between probe and target may range, sharing at least about50%, 55%, 60%, 65%, 70%, 75%, 80%, and even about 85%, 86%, 87%, 88%,89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% sequencesimilarity.

Specificity is typically the function of post-hybridization washes, thecritical factors being the ionic strength and temperature of the finalwash solution. Generally, stringent wash temperature conditions areselected to be about 5° C. to about 2° C. lower than the melting point(T_(m)) for the specific sequence at a defined ionic strength and pH.The melting point, or denaturation, of DNA occurs over a narrowtemperature range and represents the disruption of the double helix intoits complementary single strands. The process is described by thetemperature of the midpoint of transition, T_(m), which is also calledthe melting temperature. Formulas are available in the art for thedetermination of melting temperatures.

Preferred hybridization conditions for the promoter sequence of theinvention include hybridization at 42° C. in 50% (w/v) formamide, 6×SSC,0.5% (w/v) SDS, 100 μg/ml salmon sperm DNA. Exemplary low stringencywashing conditions include hybridization at 42° C. in a solution of2×SSC, 0.5% (w/v) SDS for 30 minutes and repeating. Exemplary moderatestringency conditions include a wash in 2×SSC, 0.5% (w/v) SDS at about50° C. for 30 minutes and repeating. Exemplary high stringencyconditions include a wash in 2×SSC, 0.5% (w/v) SDS, at about 65° C. for30 minutes and repeating. Sequences that correspond to the promoter ofthe present invention may be obtained using all the above conditions.For purposes of defining the invention, the high stringency conditionsare used.

Methods of alignment of sequences for comparison are well-known in theart. Optimal alignment of sequences for comparison may be conducted bythe local homology algorithm of Smith and Waterman, Adv. Appl. Math. 2:482 (1981); by the homology alignment algorithm of Needleman and Wunsch,J. Mol. Biol. 48: 443 (1970); by the search for similarity method ofPearson and Lipman, Proc. Natl. Acad. Sci. 85: 2444 (1988); bycomputerized implementations of these algorithms, including, but notlimited to: CLUSTAL in the PC/Gene program by Intelligenetics, MountainView, Calif.; GAP, BESTFIT, BLAST, FASTA, and TFASTA in the WisconsinGenetics Software Package, Genetics Computer Group (GCG), 575 ScienceDr., Madison, Wis., USA. See Higgins and Sharp, Gene 73: 237-244 (1988);Higgins and Sharp, CABIOS 5: 151-153 (1989); Corpet, et al., NucleicAcids Research 16: 10881-90 (1988); Huang, et al., Computer Applicationsin the Biosciences 8: 155-65 (1992), and Pearson, et al., Methods inMolecular Biology 24: 307-331 (1994).

Identity to the sequence of the present invention would mean apolynucleotide sequence having at least 80% sequence identity, morepreferably at least 85% sequence identity, and most preferably at least86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or100% identity, wherein the percent sequence identity is based on theentire promoter region.

Fragments of a promoter nucleotide sequence disclosed herein are alsoencompassed by this invention. Such fragments will comprise at leastabout 20 contiguous nucleotides, preferably at least about 50 contiguousnucleotides, more preferably at least about 75 contiguous nucleotides,even more preferably at least about 100 contiguous nucleotides of thepromoter nucleotide sequence disclosed herein. Such fragments willusually comprise the TATA recognition motif of the promoter sequence.Such fragments can be obtained by use of restriction enzymes to cleavethe naturally-occurring promoter nucleotide sequences disclosed herein;by synthesizing a nucleotide sequence; through the use of PCRtechnology, and the like. See particularly, Mullis et al. (1987) MethodsEnzymol. 155:335-350, and Erlich, ed. (1989) PCR Technology (StocktonPress, New York).

Such fragments encompass, for example, sequences capable of drivingtissue-preferred expression, elements responsible for temporal or tissuespecificity, elements responsive to a phytohormone, and sequences usefulas probes to identify similar sequences. Of particular interest arefragments which comprise one or more putative cis-elements, such as theas-1 like TGACG motif required for cytokinin response (Lam and Chua(1989) The Plant Cell 1:1147-1156; Benfey and Chua (1990) Science250:959-966). Three putative as-1 like motifs are present in SEQ ID NO.:4, at positions 958-962; 1334-1338; and 2408-2412.

Biologically active variants of the promoter sequence are alsoencompassed by the composition of the present invention, includingvariants resulting from site-directed mutagenesis. A regulatory“variant” is a modified form of a regulatory sequence wherein one ormore bases have been modified, removed or added. For example, a routineway to remove part of a DNA sequence is to use an exonuclease incombination with DNA amplification to produce unidirectional nesteddeletions of double stranded DNA clones. A commercial kit for thispurpose is sold under the trade name Exo-Size™ (New England Biolabs,Beverly, Mass.). Briefly, this procedure entails incubating exonucleaseIII with DNA to progressively remove nucleotides in the 3′ to 5′direction at 5′ overhangs, blunt ends or nicks in the DNA template.However, exonuclease III is unable to remove nucleotides at 3′, 4-baseoverhangs. Timed digestion of a clone with this enzyme producesunidirectional nested deletions.

One example of a regulatory sequence variant is a promoter formed by oneor more deletions from a larger promoter. The 5′ portion of a promoterup to the TATA box near the transcription start site can be deletedwithout abolishing promoter activity, as described by Zhu et al., ThePlant Cell 7: 1681-89 (1995). Such variants should retain promoteractivity, particularly the ability to drive expression in specifictissues. Promoter activity can be measured by Northern blot analysis,reporter activity measurements when using transcriptional fusions, andthe like. See, for example, Sambrook et al. (1989) Molecular Cloning: ALaboratory Manual (2nd ed. Cold Spring Harbor Laboratory, Cold SpringHarbor, N.Y.), herein incorporated by reference.

The nucleotide sequence for the promoter of the invention, as well asfragments and variants thereof, can be provided in expression cassettesalong with heterologous nucleotide sequences for expression in the plantof interest, more particularly in specific tissues of the plant. Such anexpression cassette is provided with a plurality of restriction sitesfor insertion of the nucleotide sequence to be under the transcriptionalregulation of the promoter. These expression cassettes are useful in thegenetic manipulation of any plant to achieve a desired phenotypicresponse. This may be achieved by increasing expression of endogenous orexogenous products in the specific tissues of interest. Alternatively,there may be a reduction of expression of one or more endogenousproducts, particularly enzymes or cofactors.

General categories of genes of interest for the purposes of the presentinvention include, for example, those genes involved in information,such as Zinc fingers, those involved in communication, such as kinases,and those involved in housekeeping, such as heat shock proteins.Transgenes may include genes encoding important traits for agronomicquality, insect resistance, disease resistance, herbicide resistance,and grain characteristics. Other groups of transgenes may be used toeffect expression of exogenous products such as enzymes, cofactors, andhormones from plants and other eukaryotes as well as from prokaryoticorganisms. It is recognized that any gene of interest can be operablylinked to the promoter of the invention and expressed in specifictissues.

Modifications that affect grain traits include alterations in the levelsof saturated and unsaturated fatty acids. Likewise, increased levels oflysine- and sulfur-containing amino acids may be desired, as well asmodifications of the amount and/or type of starch contained in the seed.Examples of hordothionin protein modifications are described in PCTpublications WO94/16078 dated 21 Jul. 1994 and WO96/38563 dated 5 Dec.1996; U.S. Pat. No. 5,885,801 issued Mar. 23, 1999, and U.S. Pat. No.5,703,049 issued Dec. 30, 1997; the disclosures of which areincorporated herein by reference. Additional examples are lysine- and/orsulfur-rich seed protein encoded by the soybean 2S albumin described inPCT publication WO97/35023 dated 25 Sep. 1997, and the chymotrypsininhibitor from barley, Williamson et al. (1987) Eur. J. Biochem.165:99-106, the disclosures of which are incorporated by reference.

Derivatives of the following genes can be made by site-directedmutagenesis to increase the level of preselected amino acids in theencoded polypeptide. For example, the gene encoding the barley highlysine polypeptide (BHL), is derived from barley chymotrypsin inhibitor,PCT publication WO98/20133 dated 14 May 1998. Other proteins includemethionine-rich plant proteins such as from sunflower seed, Lilley etal. (1989) Proceedings of the World Congress on Vegetable ProteinUtilization in Human Foods and Animal Feedstuffs, Applewhite, H. (ed.);American Oil Chemists Soc., Champaign, Ill.:497-502; corn, Pedersen etal. (1986) J. Biol. Chem. 261:6279; Kirihara et al. (1988) Gene 71:359,both incorporated herein by reference; and rice, Musumura et al. (1989)Plant Mol. Biol. 12:123. Other important genes encode latex, Floury 2,growth factors, seed storage factors and transcription factors.

Agronomic traits in seeds can be improved by altering expression ofgenes that affect seed growth and development during environmentalstress, Cheikh-N et al., (1994) Plant Physiol. 106(1):45-51, and genescontrolling carbohydrate metabolism to reduce seed abortion in maize,Zinselmeier et al. (1995) Plant Physiol. 107(2):385-391. In certainembodiments, the promoter of the instant invention modulates expressionof genes encoding proteins which act as cell cycle regulators, or whichaffect carbohydrate metabolism or phytohormone levels, as has been shownin tobacco and canola with other tissue-preferred promoters. (Ma, Q. H.,et al., (1998) Australian Journal of Plant Physiology 25(1): 53-59;Roeckel, P., et al., (1997) Transgenic Research 6(2):133-141.)Expression of heterologous nucleotides under the direction of thepromoter may result in maintenance of a desirable phenotype that mightotherwise be altered under adverse environmental conditions.

Insect resistance genes may encode resistance to pests that reduceyield, such as rootworm, cutworm, European corn borer, and the like.Such genes include, for example, Bacillus thuringiensis endotoxin genes,U.S. Pat. Nos. 5,366,892; 5,747,450; 5,737,514; 5,723,756; 5,593,881;Geiser et al. (1986) Gene 48:109; lectins, Van Damme et al. (1994) PlantMol. Biol. 24:825; and the like.

Genes encoding disease resistance traits include detoxification genes,such as against fumonisin (PCT publication WO96/06175, dated 29 Feb.1996); avirulence (avr) and disease resistance (R) genes (Jones et al.(1994) Science 266:789; Martin et al. (1993) Science 262:1432; Mindrinoset al. (1994) Cell 78:1089); and the like.

Alterations in gene expression may also affect the type or amount ofproducts of commercial interest; for example, starch for the productionof paper, textiles and ethanol. Another important commercial use oftransformed plants is the production of polymers and bioplastics such asis described in U.S. Pat. No. 5,602,321 issued Feb. 11, 1997. Genes suchas B-Ketothiolase, PHBase (polyhydroxybutyrate synthase) andacetoacetyl-CoA reductase (see Schubert et al. (1988) J. Bacteriol170(12):5837-5847) facilitate expression of polyhyroxyalkanoates (PHAs).

The nucleotide sequence operably linked to the promoter disclosed hereincan be an antisense sequence for a targeted gene. By “antisensesequence” is intended a nucleotide sequence that is in inverseorientation to the 5′-to-3′ normal orientation of that nucleotidesequence. When delivered into a plant cell, expression of the antisenseDNA sequence prevents normal expression of the DNA nucleotide sequencefor the targeted gene. The antisense nucleotide sequence encodes an RNAtranscript that is complementary to and capable of hybridizing with theendogenous messenger RNA (mRNA) produced by transcription of the DNAnucleotide sequence for the targeted gene. In this case, production ofthe native protein encoded by the targeted gene is inhibited to achievea desired phenotypic response. Thus the promoter sequence disclosedherein can be operably linked to antisense DNA sequences to reduce orinhibit expression of a native protein in the plant.

The expression cassette may also include, at the 3′ terminus of theheterologous nucleotide sequence of interest, a transcriptional andtranslational termination region functional in plants. The terminationregion can be native with the promoter nucleotide sequence of thepresent invention, can be native with the DNA sequence of interest, orcan be derived from another source. Convenient termination regions areavailable from the Ti-plasmid of A. tumefaciens, such as the octopinesynthase and nopaline synthase termination regions. See also, Guerineauet al. (1991) Mol. Gen. Genet. 262:141-144; Proudfoot (1991) Cell64:671-674; Sanfacon et al. (1991) Genes Dev. 5:141-149; Mogen et al.(1990) Plant Cell 2:1261-1272; Munroe et al. (1990) Gene 91:151-158;Ballas et al. 1989) Nucleic Acids Res. 17:7891-7903; Joshi et al. (1987)Nucleic Acid Res. 15:9627-9639.

The expression cassettes can additionally contain 5′ leader sequences.Such leader sequences can act to enhance translation. Translationleaders are known in the art and include: picornavirus leaders, forexample, EMCV leader (Encephalomyocarditis 5′ noncoding region),Elroy-Stein et al. (1989) Proc. Nat. Acad. Sci. USA 86:6126-6130;potyvirus leaders, for example, TEV leader (Tobacco Etch Virus), Allisonet al. (1986); MDMV leader (Maize Dwarf Mosaic Virus), Virology154:9-20; human immunoglobulin heavy-chain binding protein (BiP),Macejak et al. (1991) Nature 353:90-94; untranslated leader from thecoat protein mRNA of alfalfa mosaic virus (AMV RNA 4), Jobling et al.(1987) Nature 325:622-625); tobacco mosaic virus leader (TMV), Gallie etal. (1989) Molecular Biology of RNA, pages 237-256; and maize chloroticmottle virus leader (MCMV) Lommel et al. (1991) Virology 81:382-385. Seealso Della-Cioppa et al. (1987) Plant Physiology 84:965-968. Thecassette can also contain sequences that enhance translation and/or mRNAstability such as introns.

In those instances where it is desirable to have the expressed productof the heterologous nucleotide sequence directed to a particularorganelle, particularly the plastid, amyloplast, or to the endoplasmicreticulum, or secreted at the cell's surface or extracellularly, theexpression cassette can further comprise a coding sequence for a transitpeptide. Such transit peptides are well known in the art and include,but are not limited to, the transit peptide for the acyl carrierprotein, the small subunit of RUBISCO, plant EPSP synthase, and thelike.

In preparing the expression cassette, the various DNA fragments can bemanipulated, so as to provide for the DNA sequences in the properorientation and, as appropriate, in the proper reading frame. Towardthis end, adapters or linkers can be employed to join the DNA fragmentsor other manipulations can be involved to provide for convenientrestriction sites, removal of superfluous DNA, removal of restrictionsites, or the like. For this purpose, in vitro mutagenesis, primerrepair, restriction digests, annealing, and resubstitutions, such astransitions and transversions, can be involved.

As noted herein, the present invention provides vectors capable ofexpressing genes of interest under the control of the promoter. Ingeneral, the vectors should be functional in plant cells. At times, itmay be preferable to have vectors that are functional in E. coli (e.g.,production of protein for raising antibodies, DNA sequence analysis,construction of inserts, obtaining quantities of nucleic acids). Vectorsand procedures for cloning and expression in E. coli are discussed inSambrook et al. (supra).

In certain embodiments the nucleic acid sequences of the presentinvention can be stacked with any combination of polynucleotidesequences of interest in order to create a plant with a desiredphenotype with respect to one or more traits. For example, a hairpinconstruct of Example 6 may be “stacked” with a ckx1-2::csg constructsuch as is described in Example 5. The combinations generated mayinclude multiple copies of any one or more of the polynucleotides ofinterest.

These stacked combinations can be created by any method including butnot limited to cross breeding plants by any conventional or TopCrossmethodology, or genetic transformation. If the traits are stacked bygenetically transforming the plants, the polynucleotide sequences ofinterest can be combined at any time and in any order. For example, atransgenic plant comprising one or more desired traits can be used asthe target to introduce further traits by subsequent transformation. Thetraits can be introduced simultaneously in a co-transformation protocolwith the polynucleotides of interest provided by any combination oftransformation cassettes. For example, if two sequences will beintroduced, the two sequences can be contained in separatetransformation cassettes (trans) or contained on the same transformationcassette (cis). Expression of the sequences can be driven by the samepromoter or by different promoters. In certain cases, it may bedesirable to introduce a transformation cassette that will suppress theexpression of a polynucleotide of interest. This may be combined withany combination of other suppression cassettes or overexpressioncassettes to generate the desired combination of traits in the plant.

Vectors that are functional in plants can be binary plasmids derivedfrom Agrobacterium. Such vectors are capable of transforming plantcells. These vectors contain left and right border sequences that arerequired for integration into the host (plant) chromosome. At a minimum,between these border sequences is the gene to be expressed under controlof the promoter. In certain embodiments, a selectable marker and areporter gene are also included. For ease of obtaining sufficientquantities of vector, a bacterial origin that allows replication in E.coli is preferred.

Reporter genes can be included in the transformation vectors. Examplesof suitable reporter genes known in the art can be found in, forexample, Jefferson et al. (1991) in Plant Molecular Biology Manual, ed.Gelvin et al. (Kluwer Academic Publishers), pp. 1-33; DeWet et al.(1987) Mol. Cell. Biol. 7:725-737; Goff et al. (1990) EMBO J.9:2517-2522; Kain et al. (1995) BioTechniques 19:650-655; and Chiu etal. (1996) Current Biology 6:325-330.

Selectable marker genes for selection of transformed cells or tissuescan be included in the transformation vectors. These can include genesthat confer antibiotic resistance or resistance to herbicides. Examplesof suitable selectable marker genes include, but are not limited to,genes encoding resistance to chloramphenicol, Herrera Estrella et al.(1983) EMBO J. 2:987-992; methotrexate, Herrera Estrella et al. (1983)Nature 303:209-213; Meijer et al. (1991) Plant Mol. Biol. 16:807-820;hygromycin, Waldron et al. (1985) Plant Mol. Biol. 5:103-108; Zhijian etal. (1995) Plant Science 108:219-227; streptomycin, Jones et al. (1987)Mol. Gen. Genet. 210:86-91; spectinomycin, Bretagne-Sagnard et al.(1996) Transgenic Res. 5:131-137; bleomycin, Hille et al. (1990) PlantMol. Biol. 7:171-176; sulfonamide, Guerineau et al. (1990) Plant Mol.Biol. 15:127-136; bromoxynil, Stalker et al. (1988) Science 242:419-423;glyphosate, Shaw et al. (1986) Science 233:478-481; phosphinothricin,DeBlock et al. (1987) EMBO J. 6:2513-2518.

Other genes that could serve utility in the recovery of transgenicevents but might not be required in the final product would include, butare not limited to, examples such as GUS (β-glucuronidase), Jefferson(1987) Plant Mol. Biol. Rep. 5:387); GFP (green florescence protein),Chalfie et al. (1994) Science 263:802; luciferase, Teeri et al. (1989)EMBO J. 8:343; and the maize genes encoding for anthocyanin production,Ludwig et al. (1990) Science 247:449.

The transformation vector comprising the particular promoter sequence ofthe present invention, operably linked to a heterologous nucleotidesequence of interest in an expression cassette, can be used to transformany plant. In this manner, genetically modified plants, plant cells,plant tissue, seed, and the like can be obtained. Transformationprotocols can vary depending on the type of plant or plant cell, i.e.,monocot or dicot, targeted for transformation. Suitable methods oftransforming plant cells include microinjection, Crossway et al. (1986)Biotechniques 4:320-334; electroporation, Riggs et al. (1986) Proc.Natl. Acad. Sci. USA 83:5602-5606; Agrobacterium-mediatedtransformation, see for example, Townsend et al. U.S. Pat. No.5,563,055; direct gene transfer, Paszkowski et al. (1984) EMBO J.3:2717-2722; and ballistic particle acceleration, see for example,Sanford et al. U.S. Pat. No. 4,945,050; Tomes et al. (1995) in PlantCell, Tissue, and Organ Culture: Fundamental Methods, ed. Gamborg andPhillips (Springer-Verlag, Berlin); and McCabe et al. (1988)Biotechnology 6:923-926. Also see Weissinger et al. (1988) Annual Rev.Genet. 22:421-477; Sanford et al. (1987) Particulate Science andTechnology 5:27-37 (onion); Christou et al. (1988) Plant Physiol.87:671-674 (soybean); McCabe et al. (1988) Bio/Technology 6:923-926(soybean); Datta et al. (1990) Biotechnology 8:736-740 (rice); Klein etal. (1988) Proc. Natl. Acad. Sci. USA 85:4305-4309 (maize); Klein et al.(1988) Biotechnology 6:559-563 (maize); Klein et al. (1988) PlantPhysiol. 91:440-444 (maize); Fromm et al. (1990) Biotechnology8:833-839; Hooydaas-Van Slogteren et al. (1984) Nature (London)311:763-764; Bytebier et al. (1987) Proc. Natl. Acad. Sci. USA84:5345-5349 (Liliaceae); De Wet et al. (1985) in The ExperimentalManipulation of Ovule Tissues, ed. G. P. Chapman et al. (Longman, NewYork), pp. 197-209 (pollen); Kaeppler et al. (1990) Plant Cell Reports9:415-418; and Kaeppler et al. (1992) Theor. Appl. Genet. 84:560-566(whisker-mediated transformation); D. Halluin et al. (1992) Plant Cell4:1495-1505 (electroporation); Li et al. (1993) Plant Cell Reports12:250-255 and Christou et al. (1995) Annals of Botany 75:407-413(rice); Osjoda et al. (1996) Nature Biotechnology 14:745-750 (maize viaAgrobacterium tumefaciens); all of which are herein incorporated byreference.

The cells that have been transformed can be grown into plants inaccordance with conventional ways. See, for example, McCormick et al.(1986) Plant Cell Reports 5:81-84. These plants can then be grown, andpollinated with the same transformed strain or different strains. Theresulting plant having appropriate expression of the desired phenotypiccharacteristic can then be identified. Two or more generations can begrown to ensure that the desired phenotype is stably maintained andinherited.

All publications and patent applications mentioned in the specificationare indicative of the level of those skilled in the art to which thisinvention pertains. All publications and patent applications herein arehereby incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

The following examples are offered by way of illustration and not by wayof limitation.

EXAMPLE 1 In Situ Localization of Cytokinin Oxidase mRNA in SelectedMaize Tissues

To determine expression patterns of cytokinin oxidase (cytox) in maize,in situ hybridization was performed using the protocol of Jackson, D. P.(1991) (In situ Hybridization in Plants, Molecular Plant Pathology: APractical Approach, D. J. Bowles, S. J. Gurr, and M. McPherson, eds.Oxford University Press, England, pp. 63-74). Sense and antisense mRNAprobes of about 1 kb corresponding to cytox cDNA were labelednon-isotopically with digoxigenin and incubated with fixed sections ofmaize tissues from immature ear (7 weeks after planting), kernel atsilking (9 weeks after planting), kernels at 8 and 12 days afterpollination (DAP), primary root tip at the V6 stage, and premeioticanthers. Following extensive washing to remove unbound probe, sectionswere incubated with alkaline phosphatase-conjugated anti-digoxigeninantibody to detect areas of probe hybridization. Cytox mRNA was detectedspecifically with the antisense probe; the sense probe did nothybridize, therefore serving as a negative control.

In immature ears, there is low-level expression throughout the eartissue and developing flower. Expression is slightly elevated in thetissue subtending the flower (pedicel region), in vascular bundles ofthe ear and silk, megaspore mother cell (developing embryo sac) andrudimentary anthers.

Two weeks later, at the time of silking, expression is still low in thedeveloping ovule (nucellus and upper carpel walls), but is stronglyelevated in pedicel and part of the vascular bundles in the pedicel, aswell as in glumes.

In kernels at 8 DAP, cytox is strongly expressed in the pericarp,nucellus, pedicel, and endosperm (except the basal endosperm transferregion). It is weak or absent in the embryo and placental-chalazalregion. Some regions of the pedicel show lower levels of expression,whereas the vascular strands show a strong signal. Analysis oftransverse sections probed with cytox reveal that the cells of thevascular bundles that show strong signal are protoxylem and protophloemelements. Metaxylem and metaphloem (the more mature elements) show nosignal.

In 12 DAP kernels, strong cytox expression is observed in the pedicel(especially vascular bundles) and intermediate expression in pericarpand endosperm. Expression is low in embryo, basal part of endosperm andplacental-chalazal region.

In primary roots, strong cytox expression is seen in all tissues of thecell elongation zone, starting about 6 mm above the root tip. Signal isstrong in vascular bundles, even in those that extend into the region oflower signal towards the root tip. Signal is low or absent in rootmeristem and root cap, except for the outermost layer (epidermis).

In premeiotic anthers, a strong signal is observed in the glumes and theouter layers of the anther wall. The signal is weaker in tapetum andmicrospores.

EXAMPLE 2 Generation of Transgenic Events Via AgrobacteriumTransformation

The Agrobacterium strain utilized in this example was modified tocontain nucleic acid encoding the cytokinin oxidase promoter and a GUSreporter gene to be expressed in the transformed cells.

The vectors of this example were constructed using standard molecularbiology techniques known to those of ordinary skill in the art. Areporter gene and a selectable marker gene were inserted between theT-DNA borders of a superbinary vector. The reporter gene was theb-glucuronidase (GUS) gene (Jefferson, R. A. et al., 1986, Proc. Natl.Acad. Sci. (USA) 83:8447-8451) into whose coding region was inserted thesecond intron from the potato ST-LS1 gene (Vancanneyt et al., Mol. Gen.Genet. 220:245-250, 1990), to produce intron-GUS, in order to preventexpression of the gene in Agrobacterium (see Ohta, S. et al., 1990,Plant Cell Physiol. 31(6):805-813). A fragment containing bases 2 to 310from the terminator of the potato proteinase inhibitor (pinII) gene (Anet al., Plant Cell 1:115-122, 1989) was blunt-end ligated downstream ofthe GUS coding sequence, to create the GUS expression cassette.

For the selectable marker, a Cauliflower Mosaic Virus 35S promoter witha duplicated enhancer region (2X35S; bases −421 to −90 and −421 to +2from Gardner et al., Nucl. Acids Res. 9:2871-2888, 1981) was created. Afragment containing the first intron of the maize alcohol dehydrogenasegene ADH1-S (Dennis et al., Nucl. Acids Res. 12:3983-3990, 1984) wasinserted downstream of the 35S promoter. The BAR coding sequence(Thompson et al., EMBO J. 6:2519-2523, 1987) was cloned downstream ofthe leader sequence, with the pinII terminator ligated downstream ofBAR, to create the BAR expression cassette.

In summary, the plasmid was constructed by inserting the GUS expressioncassette and the BAR expression cassette between the right and leftT-DNA borders in pSB11. The GUS cassette was inserted proximal to theright T-DNA border. The ckx1-2 promoter fragment (SEQ ID NO: 1) wasinserted into the vector in front of the intron-GUS gene. The plasmidpSB11 was obtained from Japan Tobacco Inc. (Tokyo, Japan). Theconstruction of pSB11 from pSB21 and the construction of pSB21 fromstarting vectors has been described by Komari et al. (1996, Plant J.10:165-174). The T-DNA of this plasmid was integrated into thesuperbinary plasmid pSB1 (Saito et al., EP 672 752 A1) by homologousrecombination between the two plasmids. The plasmid pSB1 was alsoobtained from Japan Tobacco Inc. E. coli strain HB101 containing theplasmid containing the ckx1-2 promoter (SEQ ID NO: 1) was mated withAgrobacterium strain LBA4404 harboring pSB1 to create the cointegrateplasmid in the Agrobacterium tumefaciens strain LBA4404 using the methodof Ditta et al. (Proc. Natl. Acad. Sci. USA 77:7347-7351, 1980). (Seealso U.S. Pat. No. 5,981,840 for a further discussion ofAgrobacterium-mediated transformation.)

The resulting co-integrated plasmid, the product from the tri-parentalmating described above, was transformed into the genotypes (1) Hi-II and(2) Hi-II×PHN46. (See U.S. Pat. No. 5,567,861 for more information aboutPHN46.) TO plants were generated and promoter analysis was conducted onT1 seed from both genotypes.

EXAMPLE 3 Expression of a Cytokinin Oxidase Promoter-Gus FusionConstruct in Transgenic Maize

The ckx1-2 promoter of SEQ ID NO: 1 was isolated using the GenomeWalker™ cloning kit with genomic DNA from the inbred B73. The procedurefor promoter isolation is described in the User Manual for the GenomeWalker™ cloning kit sold by Clontech Laboratories, Inc., Palo Alto,Calif. (Genome Walker™ User Manual Clontech PT3042-1 version PR68687)

The promoter was fused to GUS. Maize embryos were transformed with theckx1-2 (SEQ ID NO: 1): GUS construct via Agrobacterium. Afterregeneration, leaf discs of 8-week old T0 plants were assayed for GUSactivity. GUS staining was detected in three independent transgenicevents and the event showing the strongest signal was used for furtheranalysis. Strong GUS staining was found in the leaf midrib and in smalland large vascular strands (FIG. 1A). In the large vascular strands,label was confined to protoxylem and bundle sheath cells (FIG. 1B). Noor little staining was observed in phloem tissue, epidermis or cortex.As shown in FIGS. 1C and D, vascular staining was also observed incoleoptile sections of T1 seedlings. In secondary roots of the sameseedlings, staining was primarily found in the vasculature (FIG. 1E).Staining was stronger in the root elongation zone and decreased in theupper region of the root. Weak or no staining was detected in thevasculature of primary roots but occasional pigmentation was detected indeveloping root primordia (FIG. 1F). The ckx1-2 promoter (SEQ ID NO: 1)was also found to be active in the base of tassel spikelets, pedicels of8 DAP kernels, and in the vasculature of tassel spikelet glumes (datanot shown).

Based on the analysis of ckx1-2 (SEQ ID NO.: 1): GUS transgenic plants,several constructs were generated for transformation into maize. Theseconstructs contained the ckx1-2 promoter fused to polynucleotides thatencode proteins involved in the synthesis of cytokinins and thestimulation of the cell cycle.

EXAMPLE 4 Isolation of Additional 5′ Sequence

A maize bacterial artificial chromosome (BAC) library derived frominbred Mo17 was screened using a probe representing the ckx1-2 promotersequence previously identified (see SEQ ID NO: 1). The Qiagen®Large-Construct Kit (Qiagen, Inc., 27220 Turnberry Lane, Valencia,Calif. 91355) was used according to the manufacurer's instructions toisolate BAC DNA from positive clones. Using the GenomeWalker™ cloningkit (Clontech Laboratories, Inc., Palo Alto, Calif.) according to themanufacturer's instructions, and appropriate sequence-specific primers(see SEQ ID NO: 5-9), 3.3. kb of sequence 5′ to the ckx1-2 codingsequence was amplified and sequenced (SEQ ID NO: 4).

EXAMPLE 5 Use of Cytokinin Oxidase Promoter in Regulating CytokininBiosynthesis

An example of one predicted phenotype of a transgenic event containing ackx1-2::csg (cytokinin synthesis gene) construct is described here.External application of cytokinins has previously been shown tostimulate cytokinin oxidase activity in plants. Using a leaf disc assayand in vitro kernel culture, it has been demonstrated that thisinduction occurs at the transcript level, which likely means thatcytokinin application causes an increase in cytokinin oxidase promoteractivity. Research also showed, in vivo, that in developing kernels,cytokinin oxidase transcripts accumulate in response to an increase incytokinin levels. It has been demonstrated in numerous experiments thatapplication of cytokinins enhances sink strength. In addition, there isexperimental evidence which indicates that abiotic stress (in particularheat stress) causes an elevation in cytokinin oxidase message indeveloping kernels. Thus, it is predicted that under normalenvironmental conditions or transient abiotic stress conditions,cytokinin levels in sink tissues (such as developing kernels) ofckx1-2::csg events would maintain sink activity (cell division,amyloplast differentiation, etc.) with either an enhanced or stabilizedgrain yield. The interest in using this promoter also derives from thefact that, since it reacts to cytokinin in vivo, it most likely directsexpression in or around the tissue where natural cytokinin biosynthesisoccurs in plants. Based on work in Urtica dioica, Pisum sativum andmaize, it has been shown that cytokinins are translocated in the plantvia the xylem. Interestingly, a mutant in a cytokinin receptor-kinase(Cre1) has been shown to display abnormal vascular differentiation inArabidopsis. Thus, one would predict that maize events containing ackx1-2::csg construct would also produce more cytokinins in leaf androot vasculature. This synthesis of cytokinins would stimulate theckx1-2 promoter, which in turn would initiate more cytokinin production.In summary, the overall elevation in cytokinins in the plant would bothincrease the sink strength of a particular organ (such as the kernel)and enhance overall growth of the plant by stimulating vascular celldivision and/or differentiation.

EXAMPLE 6 Downregulation of Cytokinin Catabolism

The ckx1-2 promoter of the present invention can be used in constructsdesigned to downregulate cytokinin oxidase. For example, certainembodiments comprise a construct comprising a segment of the ckx1-2promoter such that, upon expression, self-hybridization of the RNAresults in formation of hairpin RNA (hpRNA), resulting intranscriptional gene silencing of the native cytokinin oxidase gene.That is, the embodiment comprises a nucleotide sequence which, whenexpressed in a cell, forms a hairpin RNA molecule (hpRNA), whichsuppresses (i.e., reduces or eliminates) expression of the endogenouscytokinin oxidase gene from its endogenous promoter. The ability ofhpRNAs to suppress expression of a gene has been described (see, e.g.,Matzke et al. (2001) Curr. Opin. Genet. Devel. 11:221-227; Scheid et al.(2002) Proc. Natl. Acad. Sci., USA 99:13659-13662; Waterhouse andHelliwell (2003) Nature Reviews Genetics 4:29-38; Aufsaftz et al (2002)Proc. Nat'l. Acad. Sci. 99(4):16499-16506; Sijen et al., Curr. Biol.(2001) 11:436-440).

The promoter which is operably linked to the nucleotide sequenceencoding the hpRNA can be any promoter that is active in plant cells,particularly a promoter that is active (or can be activated) inreproductive tissues of a plant. As such, the promoter can be, forexample, a constitutively active promoter, an inducible promoter, atissue-specific promoter, a tissue-preferred promoter, a developmentalstage specific promoter, or a developmental stage preferred promoter.

A hairpin may target a single promoter or may target two or morepromoters by means of a single transcribed RNA. The hairpin-encodingregion may be located in any appropriate position within the construct,such as within an intron of an encoded gene or within 5′ or 3′non-coding regions, or may be the sole expressed element of theconstruct.

Methods for preparing said constructs and transforming plants may be aspreviously described (for example, see Cigan et al., Sex Plant Reprod.14:135-142, 2001).

Said constructs for downregulating cytokinin oxidase expression may beused in combination with constructs or methods to increase cytokininbiosynthesis activity, such as those described in Example 5.

1. An expression cassette comprising a fragment of at least 21consecutive nucleotides of SEQ ID NO: 4 operably linked to a promoter.2. A plant, or its parts, stably transformed with the expressioncassette of claim
 1. 3. The plant of claim 2, wherein expression of saidfragment results in downregulation of expression of cytokinin oxidase.4. The plant of claim 2, wherein said plant is maize, barley, wheat,oat, rye, sorghum or rice.
 5. The plant of claim 2, wherein said plantis soybean, alfalfa, safflower, tobacco, sunflower, cotton, or canola.6. A method of downregulating expression of cytokinin oxidase in aplant, comprising transforming a plant cell with the expression cassetteof claim 1 and regenerating a stably transformed plant from saidtransformed cell.
 7. The expression cassette of claim 1 in which thefragment comprises at least 21 consecutive nucleotides of positions1-1832 of SEQ ID NO:
 4. 8. The method of claim 6, wherein (a) expressionof native cytokinin oxidase is downregulated and (b) expression oftransgenic cytokinin oxidase is not downregulated.